## Lave Carson, age 14, of Prescott, Ariz., for his question:

# How far does earth's gravity extend?

In theory, the gravity of the earth reaches out to infinity ‑ which is further than our imaginations can reach. Gravitation is a force present in every speck of matter, every object, every heavenly body. Every object is attracted to every other object by this force of gravitation.

The more mass, or matter, an object has the greater its force of gravitation. The earth's gravitation is far less strong than the gravitation of the sun. For the sun has 333,400 times more mass, or weight, than the earth. Mass, however, is only part of the story of gravitation. Distance also plays a role.

Gravity, strictly speaking, is the force of gravitation in operation on the surface of the earth or any other heavenly body. It pulls at every object on the face of the earth, and it is this force which gives weight to objects. If earth's gravity were less, then every object on earth would. weigh less. And, with distance, earth's gravity does diminish.

When we think of gravity and the weight it gives, there are always two objects involved. For remember, the force of gravitation is present in every particle of matter in the universe.

Let's say you weigh 120 pounds. This means that the mass, or bulk, of matter in your body and the mass of the earth combine to give you a weight of 120 pounds at the earth's surface. The strange force seems to operate from the center of your body's mass to the center of the earth's mass. And the force decreases when these two points come further apart,

This decrease in gravity works to an exact scale. When the distance between two bodies is doubled, the force of gravity between them becomes one fourth. On the earth's surface you are about 4,000 miles from the earth's center of gravity. Double this distance by flying up 4,000 miles above the earth's surface and the gravity of the earth affects you only one fourth. In one fourth. In other words, you weigh only 30 pounds.

Double the distance again and you weigh less than eight pounds. Your weight is reduced by three quarters every time you double the distance between you and the center of the earth. True, a million miles out in space you would feel hardly any gravity from the earth. Even so, fine instruments might detect it. For there is no point at which the earth's gravity comes to a sudden end. In theory it must go on to infinity.

Out in space, far from any heavenly body, gravity is weak. Therefore, the weight, say, of a space traveler is reduced to almost nothing. His body becomes practically weightless.